The present invention relates to a method for inactivating Cryptosporidium parvum in water and in particular to a method for the prevention of Cryptosporidium parvum and other protozoans, such as Giardia muris, from establishing infection in human hosts, as measured by the ability to infect neo-natal mice, using low doses of ultraviolet light.
It has been generally well recognized that it is necessary to kill or inactivate protozoan oocysts so that they cannot infect susceptible hosts. This is especially important in drinking water. One such method is the use of ultraviolet (xe2x80x9cUVxe2x80x9d) light. The prior art teaches that a UV dose of at least 3000 mJ/cm2 is required to inactivate Cryptosporidium parvum (Lorenzo-Lorenzo et al., J. Parasitol. 1993, 79, 67-70) and Giardia muris (E. L. Jarol, xe2x80x9cEffect of Disinfectants on Giardia Cystsxe2x80x9d, CRC Critical Reviews in Environmental Control, 1988, 18, 1-28). Snowball and coworkers (UK Patent Application #9416287.2, Nov. 8, 1984; Wat. Res., 1995, 29, 2583-2586) developed an apparatus that first filtered out Cryptosporidium oocysts and then exposed them to UV doses of 700-800 mJ/cm2. The patent teaches the use of 2 xcexcm screen filters to trap Cryptosporidium oocysts which are then irradiated with a bank of low-pressure Hg lamps for a UV dose of 350-400 mJ/cm2. The filter is then backwashed onto a second filter and the irradiation is repeated for a total dose of 700-800 mJ/cm2. The patent discloses that the treatment xe2x80x9ckillsxe2x80x9d the organisms.
M. J. Lorenzo-Lorenzo, M. E. Area-Mazea, I. Villacorta-Martinez de Maturana and D. Duran-Oreiro [xe2x80x9cEffect of Ultraviolet Disinfection of Drinking Water on the Viability of Cryptosporidium parvum Oocystsxe2x80x9d, J. Parasitol. 1993, 79(1), 67-70] report the prevention of infection in mice after exposure to at least 150 min. of UV from a (presumably) low-pressure Hg lamp. Although the paper is not clear, it can be inferred that the UV dose applied was over 5000 mJ/cm2 to obtain better than 2 logs reduction in infectivity. The authors claim that exposure to UV for 150 min. or more xe2x80x9celiminatesxe2x80x9d infectivity, but they give no mechanism other than to say xe2x80x9cUV radiation disrupts DNA by causing formation of thy[ia]mine dimers, and high levels may lead to cell deathxe2x80x9d. At the UV doses they applied, the effects observed almost certainly arose from cell death.
In a paper by A. Bushnell, W. Clark, J. Dunn and K. Salisbury [xe2x80x9cPulsed Light Sterilization of Products Packaged by Blow-Fill-Seal Techniquesxe2x80x9d, Pharm. Engin. 1997, September/October, 74-83], a pulsed UV technique for xe2x80x9csterilizingxe2x80x9d surfaces containing bacteria, fungi, spores, viruses, protozoa and oocysts is described. The required UV doses were reported to be over 1000 mJ/cm2. The effectiveness of the method was assayed using mouse infectivity. At the reported UV doses, the effects were believed to be due to cell death.
In a paper by R. LaFrenz [xe2x80x9cHigh Intensity Pulsed UV for Drinking Water Treatmentxe2x80x9d, Proc. AWWA WQTC Conference, Denver, Colo., November, 1997], a similar pulsed system was described. While very few details were given, it appears that mouse infectivity assay was used and 6 logs of xe2x80x9cinactivationxe2x80x9d of Cryptosporidium was obtained at energy levels of approximately 200 mJ/cm2 and greater. The paper claims that the pulsed UV overcomes the xe2x80x9cDNA repair mechanismxe2x80x9d; however, the UV doses applied are much larger than required with either a steady-state medium pressure or low pressure Hg lamp, as shown herein.
From the references cited above, we infer that the prior art teaches that very large UV doses ( greater than 200 mJ/cm2 and up to 5000 mJ/cm2) are required to inactivate Cryptosporidium by xe2x80x9ckillingxe2x80x9d the organisms. Accordingly, it is an object of the invention to provide a method using ultraviolet light to treat water in an effective way so that Cryptosporidium oocysts cannot infect susceptible hosts or, in other words, to xe2x80x9cdisinfectxe2x80x9d the water in regard to Cryptosporidium oocysts that may be present. It is another object of the invention to provide a method using ultraviolet light from a medium-pressure mercury lamp to render the Cryptosporidium oocysts unable to infect. It is yet another object of the present invention to provide a method using ultraviolet light that is cost-effective in treating drinking water to eliminate the potential for infection by Cryptosporidium oocysts and Giardia cysts. The final object of the invention is to provide a method using ultraviolet light from a low-pressure mercury lamp to render Cryptosporidium oocysts and Giardia cysts unable to infect.
Generally it has been discovered that it is not necessary to xe2x80x9ckillxe2x80x9d pathogens, such as Cryptosporidium parvum or Giardia muris with ultraviolet light in order to prevent infection; one need only apply enough ultraviolet light to prevent the organism from xe2x80x9creplicatingxe2x80x9d. The method of the present invention prevents replication (cell mitosis) by inactivating the DNA to prevent infection. The UV doses required to prevent replication are orders of magnitude lower than required to xe2x80x9ckillxe2x80x9d the oocysts. This means that the cost of UV treatment to prevent infection by Cryptosporidium oocysts will be markedly lower.
It has been found that when biological organisms are exposed to ultraviolet light (UV) in the range of 200-300 nm, the UV can be absorbed by DNA, RNA, and proteins. Absorption by proteins can lead to rupture of cell walls and death of the organism. Absorption by DNA or RNA (specifically by thymine bases) is known to cause inactivation of the DNA or RNA double helix strands through the formation of thymine dimers. If enough of these dimers are created in DNA, the DNA replication process is disrupted and hence, in mitosis, the cell cannot replicate. Cells that cannot replicate cannot infect. The present invention utilizes UV doses substantially lower (to achieve the state of hindered replication) by orders of magnitude than those required to cause oocyst death.
The present invention preferably utilizes a broad band (200-300 nm) medium-pressure mercury UV lamp to achieve the disinfection. In another embodiment of the invention, a low-pressure mercury (essentially monochromatic) UV lamp can be used. The dose required with a medium-pressure lamp was measured to be 11 mJ/cm2 to achieve better than 5.9 log disinfection. From this it can be inferred that a dose of 7 mJ/cm2 will achieve better than 4 log disinfection (99.99%) and 3.6 mJ/cm2 will achieve better than 2 log disinfection (99%). For low pressure lamps a dose of 8 and 16 mJ/cm2 was required to achieve 4.1 and 4.3 log disinfection, respectively. Thus, the dose levels of UV are significantly lower than those used before resulting in significantly lower power levels needed to achieve the results. It has been found that inactivation of Cryptosporidium and similar organisms such as Giardia occurs at dosages from about 1 mJ/cm2. Accordingly, the method provides a substantial improvement in the cost effectiveness of UV for the disinfection of contaminated drinking water as regards to Cryptosporidium oocysts and Giardia cysts that may be present. Other advantages will become apparent from a perusal of the following detailed description of a presently preferred embodiment of the invention.